linux-kernel - Re: [PATCH] powerpc/kprobes: Fix trap address when trap happened in real mode

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Date:   Mon, 17 Feb 2020 16:38:50 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Larry Finger <Larry.Finger@...inger.net>,
        "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        stable@...nel.vger.org,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] powerpc/kprobes: Fix trap address when trap happened in
 real mode



Le 17/02/2020 à 11:27, Masami Hiramatsu a écrit :
> On Mon, 17 Feb 2020 10:03:22 +0100
> Christophe Leroy <christophe.leroy@....fr> wrote:
> 
>>
>>
>> Le 16/02/2020 à 13:34, Masami Hiramatsu a écrit :
>>> On Sat, 15 Feb 2020 11:28:49 +0100
>>> Christophe Leroy <christophe.leroy@....fr> wrote:
>>>
>>>> Hi,
>>>>
>>>> Le 14/02/2020 à 14:54, Masami Hiramatsu a écrit :
>>>>> Hi,
>>>>>
>>>>> On Fri, 14 Feb 2020 12:47:49 +0000 (UTC)
>>>>> Christophe Leroy <christophe.leroy@....fr> wrote:
>>>>>
>>>>>> When a program check exception happens while MMU translation is
>>>>>> disabled, following Oops happens in kprobe_handler() in the following
>>>>>> test:
>>>>>>
>>>>>> 		} else if (*addr != BREAKPOINT_INSTRUCTION) {
>>>>>
>>>>> Thanks for the report and patch. I'm not so sure about powerpc implementation
>>>>> but at where the MMU translation is disabled, can the handler work correctly?
>>>>> (And where did you put the probe on?)
>>>>>
>>>>> Your fix may fix this Oops, but if the handler needs special care, it is an
>>>>> option to blacklist such place (if possible).
>>>>
>>>> I guess that's another story. Here we are not talking about a place
>>>> where kprobe has been illegitimately activated, but a place where there
>>>> is a valid trap, which generated a valid 'program check exception'. And
>>>> kprobe was off at that time.
>>>
>>> Ah, I got it. It is not a kprobe breakpoint, but to check that correctly,
>>> it has to know the address where the breakpoint happens. OK.
>>>
>>>>
>>>> As any 'program check exception' due to a trap (ie a BUG_ON, a WARN_ON,
>>>> a debugger breakpoint, a perf breakpoint, etc...) calls
>>>> kprobe_handler(), kprobe_handler() must be prepared to handle the case
>>>> where the MMU translation is disabled, even if probes are not supposed
>>>> to be set for functions running with MMU translation disabled.
>>>
>>> Can't we check the MMU is disabled there (as same as checking the exception
>>> happened in user space or not)?
>>>
>>
>> What do you mean by 'there' ? At the entry of kprobe_handler() ?
>>
>> That's what my patch does, it checks whether MMU is disabled or not. If
>> it is, it converts the address to a virtual address.
>>
>> Do you mean kprobe_handler() should bail out early as it does when the
>> trap happens in user mode ?
> 
> Yes, that is what I meant.
> 
>> Of course we can do that, I don't know
>> enough about kprobe to know if kprobe_handler() should manage events
>> that happened in real-mode or just ignore them. But I tested adding an
>> event on a function that runs in real-mode, and it (now) works.
>>
>> So, what should we do really ?
> 
> I'm not sure how the powerpc kernel runs in real mode.
> But clearly, at least kprobe event can not handle that case because
> it tries to access memory by probe_kernel_read(). Unless that function
> correctly handles the address translation, I want to prohibit kprobes
> on such address.
> 
> So what I would like to see is, something like below.
> 
> diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
> index 2d27ec4feee4..4771be152416 100644
> --- a/arch/powerpc/kernel/kprobes.c
> +++ b/arch/powerpc/kernel/kprobes.c
> @@ -261,7 +261,7 @@ int kprobe_handler(struct pt_regs *regs)
>          unsigned int *addr = (unsigned int *)regs->nip;
>          struct kprobe_ctlblk *kcb;
>   
> -       if (user_mode(regs))
> +       if (user_mode(regs) || !(regs->msr & MSR_IR))
>                  return 0;
>   
>          /*
> 
> 

With this instead change of my patch, I get an Oops everytime a kprobe 
event occurs in real-mode.

This is because kprobe_handler() is now saying 'this trap doesn't belong 
to me' for a trap that has been installed by it.

So the 'program check' exception handler doesn't find the owner of the 
trap hence generate an Oops.

Even if we don't want kprobe() to proceed with the event entirely 
(allthough it works at least for simple events), I'd expect it to fail 
gracefully.

Christophe